Science before 1900: and Science after 1900:

Science before 1900: and Science after 1900:

13th century -New Universities were established in many parts of Europe.

16th century, sea adventure started from Europe to discover new places along the sea coasts.

17th century, Royal Society of London and The French Academy of sciences were founded to study the pure and applied Science.

18th century, scientists discovered the gases,like CO2, H2, O2 and Lavoisier of France correctly interpretation the process of burning,he explained the combustion theory. He also made a list of Known elements for the first time. He explained the concept of oxidation and reduction. He gave the law of conservation of mass.

Antoine Lavoisier revolutionized chemistry. He named the elements carbon, hydrogen and oxygen; discovered oxygen’s role in combustion and respiration; established that water is a compound of hydrogen and oxygen; discovered that sulfur is an element, and helped continue the transformation of chemistry from a qualitative science into a quantitative one.

In 1772 Lavoisier discovered that when phosphorus or sulfur are burned in air the products are acidic. The products also weigh more than the original phosphorus or sulfur, suggesting the elements combine with something in the air to produce acids. But what?

In 1779 Lavoisier coined the name oxygen for the element released by mercury oxide. He found oxygen made up 20 percent of air and was vital for combustion and respiration. He also concluded that when phosphorus or sulfur are burned in air, the products are formed by the reaction of these elements with oxygen.

In 1777 Lavoisier correctly identified sulfur as an element. He had carried out extensive experiments involving this substance and observed that it could not be broken down into any simpler substances.

In 1778 Lavoisier found that when mercury oxide is heated its weight decreases. The oxygen gas it releases has exactly the same weight as the weight lost by the mercury oxide.

Lavoisier announced a new fundamental law of nature: the law of conservation of mass:

• matter is conserved in chemical reactions

In 1783 Lavoisier coined the name ‘hydrogen’ for the gas which Henry Cavendish had recognized as a new element in 1766; Cavendish had called the gas inflammable air.

Lavoisier burned hydrogen with oxygen and found that water was produced, establishing that water is not an element, but is actually a compound made from the elements hydrogen and oxygen. 

In 1789 Lavoisier published his groundbreaking Elementary Treatise on Chemistry. Itcontained a list of chemical elements. The list included oxygen, nitrogen, hydrogen, sulfur, phosphorus, carbon, antimony, cobalt, copper, gold, iron, manganese, molybdenum, nickel, platinum, silver, tin, tungsten, and zinc.

Antoine Lavoisier was called the father of modern chemistry.

19th century, the concept of atom was put forth by John Dalton and he calculated the relative atomic weights of diferent elements, he compared the atomic masses, taking hydrogen atom as a unit mass.

Dalton's fascination with gases gradually led him to formally assert that every form of matter (whether solid, liquid or gas) was also made up of small individual particles. In an article he wrote for the Manchester Literary and Philosophical Society in 1803, Dalton created the first chart of atomic weights.

In 1808,In A New System of Chemical Philosophy, Dalton introduced his belief that atoms of different elements could be universally distinguished based on their varying atomic weights. In so doing, he became the first scientist to explain the behavior of atoms in terms of the measurement of weight. He also uncovered the fact that atoms couldn't be created or destroyed.

The law of definite proportion and multiple proportion was explained, relating to a chemical change.

Elements were represented by symbols and chemical reactions were written using these symbols.

Volta developed a primary battery cell by compiling a pile of dissimilar elements arranged alternately with moist cardboard separators. He could produce steady electricity. The cell was further modified by Denial for practical use.

Alessandro Volta Invented the first electric battery – which people then called the “voltaic pile” – in 1800. Using his invention, scientists were able to produce steady flows of electric current for the first time, unleashing a wave of new discoveries and technologies.

The water was decomposed using the electric energy into two gases. Water was confirmed as a compound.

Using large battery, Davy isolated new elements like K, Na Ca etc.

Davy discovered several new elements. In 1807 he electrolyzed slightly damp fused potash and then soda—substances that had previously resisted decomposition and hence were thought by some to be elements—and isolated potassium and sodium. He went on to analyze the alkaline earths, isolating magnesium, calcium, strontium, and barium.

The list of elements grew considerably.

In the year 1860, A first chemical meet was held in Germany, where 140 delegates participated to discuss the principles of chemistry.

The very first international scientific conference was held in Karlsruhe, Germany on Sept. 3, 1860.
 It was an science landmark also, essential for clearing up several major difficult problems that were blocking the advance of chemistry.
Clearing up the element sequence (using weights at that time) took place by Cannizzaro's interpretation of Avogadro's Law.

German chemist Lothar Meyer, and the Russian chemist Dmitri Mendeleev, who had both been in attendance at Karlsruhe, constructed element arrangements using the Cannizzaro numbers - on tables: with the elements arranged in rows and columns - for schoolbooks.

An important long-term result of the Karlsruhe Congress was the adoption of the now-familiar atomic weights (actually, atomic masses) of approximately 1 for hydrogen, 12 for carbon, 16 for oxygen, Cl 35.5, K39, Ca 40, Br 80, Rb 85, Sr 88, I 127, Cs 133, Ba 137 and so forth. There was also a recognition that certain elements, such as hydrogen, nitrogen, and oxygen, were composed of diatomic molecules and not individual atoms.

In 1869 Mendeleev arranged the known elements in a table of rows and columns known as periodic table of elements.

On March 6, 1869, Mendeleev made a formal presentation to the Russian Chemical Society, entitled The Dependence between the Properties of the Atomic Weights of the Elements, which described elements according to both weight and valence. This presentation stated that

1. The atomic mass, exhibit an apparent periodicity of properties.
2. Elements which are similar as regards to their chemical properties have atomic weights which are either of nearly the same value (e.g., Pt, Ir, Os) or which increase regularly (e.g., K, Rb, Cs).
3. The arrangement of the elements in groups of elements in the order of their atomic weights corresponds to their so-called valencies, as well as, to some extent, to their distinctive chemical properties; as is apparent among other series in that of Li, Be, B, C, N, O, and F.
4. The elements which are the most widely diffused have small atomic weights.
5. The magnitude of the atomic weight determines the character of the element, just as the magnitude of the molecule determines the character of a compound body.
6. We must expect the discovery of many yet unknown elements–for example, two elements, analogous to aluminium and silicon, whose atomic weights would be between 65 and 75.
7. The atomic weight of an element may sometimes be amended by a knowledge of those of its contiguous elements. Thus the atomic weight of iodine (126.9).
8. Certain characteristic properties of elements can be foretold from their atomic weights.

Mendeleev studied petroleum origin and concluded that hydrocarbons are abiogenic and form deep within the earth. He wrote: "The capital fact to note is that petroleum was born in the depths of the earth, and it is only there that we must seek its origin."

By the end of the century almost all elements were known.

The 20th century opened with alternating electric current. Alternating current (AC) has the distinct advantage over direct current (DC; a steady flow of electric charge in one direction) of being able to transmit power over large distances without great loss of energy to resistance.

Alternating current systems can use transformers to change voltage from low to high level and back, allowing generation and consumption at low voltages but transmission, possibly over great distances, at high voltage, with savings in the cost of conductors and energy losses. The AC power systems was developed and adopted rapidly after 1886 due to its ability to distribute electricity efficiently over long distances, overcoming the limitations of the direct current system.

For three-phase at utilization voltages, a four-wire system is often used. When stepping down three-phase, a transformer with a Delta (3-wire) primary and a Star (4-wire, center-earthed) secondary is often used so there is no need for a neutral on the supply side.

Electron was discovered as a subatomic particle. 

 Subatomic particles:

Electron was discovered as a subatomic particle.

 Properties of cathode ray particle

1. They travel in straight lines.

2. They are independent of the material composition of the cathode.

3. Applying electric field in the path of cathode ray deflects the ray towards positively charged plate. Hence cathode ray consists of negatively charged particles.

J. J. Thomson measured the charge-by-mass-ratio (e/m) of cathode ray particle using deflection in both electric and magnetic field. E/m = −1.76×10⁸ coulomb per gram

The cathode ray particle turned out to be 2000 times lighter than hydrogen.

In 1909, American physicist R. Millikan measured the charge of an electron using negatively charged oil droplets. The measured charge (e) of an electron is −1.60×10⁻¹⁹ Coulombs.

Using the measured charge of electron, we can calculate the mass of electron from e/m ratio given by J. J. Thomson’s cathode ray experiment.

e/m = −1.76×10⁸ Coulomb-per-gram, m = e/−1.76×108, Putting e = −1.60×10⁻¹⁹ Coulomb,

  m = 9.1×10⁻²⁸ gram.

 What we have learned

1. Electron was discovered by J. J. Thomson in Cathode Ray Tube (CRT) experiment.

2. Electrons are negatively charged particles with charge-to-mass ratio −1.76×10⁸ C/gm

3. The charge of an electron was measured by R. Millikan in Oil drop experiment.

4. Charge of an electron is −1.60×10⁻¹⁹ C

5. Mass of an electron is 9.1×10⁻²⁸ gram.

6. Electron is approximately 2000 times lighter than hydrogen.

Within the next three decades the structure of atom was completely known.

Rutherford proposed the following structural features of an atom:

1.Most of the atom’s mass and its entire positive charge are confined in a small core, called nucleus. The positively charged particle is called proton.

2.Most of the volume of an atom is empty space.

3.The number of negatively charged electrons dispersed outside the nucleus is same as number of positively charge in the nucleus. It explains the overall electrical neutrality of an atom.

But scientists soon realized that the atomic model offered by Rutherford is not complete. Various experiments showed that mass of the nucleus is approximately twice than the number of proton. What is the origin of this additional mass?

In 1930, W. Bothe and H. Becker found an electrically neutral radiation when they bombarded beryllium with alpha particle. They thought it was photons with high energy (gamma rays).

 In 1932, Irène and Frédéric Joliot-Curie showed that this ray can eject protons when it hits paraffin or H-containing compounds.

 The question arose that how mass less photon could eject protons which are 1836 times heavier than electrons. So the ejected rays in bombardment of beryllium with alpha particles cannot be photon.

 In 1932, James Chadwick performed the same experiment as Irène and Frédéric Joliot-Curie but he used many different target of bombardment besides paraffin. By analyzing the energies of different targets after bombardment he discovered the existence of a new particle which is charge less and has similar mass to proton. This particle is called neutron. Beryllium undergoes the following reaction when it is bombarded with alpha particle: Be⁹ + ᾳ⁴ → C¹² + C¹³ + ꞑ

 By this time, man could fly in the air, transport large quantities of goods seamlessly on railroads. Can instantly talk on the phone and broadcast on radio, can make cinemas and all. Men fought two world wars. They could split the atom to produce bombs and one suck atomic bomb was used against Japan by the USA in world war-II. This led to the end of wars once and for all. After world war-II, Most of the colonies have become free nations.

Relativity

Relativity

Relativity, wide-ranging physical theories formed by the German-born physicist Albert Einstein. With his theories of special relativity (1905) and general relativity (1915), Einstein overthrew many assumptions underlying earlier physical theories, redefining in the process the fundamental concepts of space, time, matter, energy, and gravity. Along with quantum mechanics, relativity is central to modern physics. In particular, relativity provides the basis for understanding cosmic processes and the geometry of the universe itself.
“Special relativity” is limited to objects that are moving with respect to inertial frames of reference—i.e, in a state of uniform motion with respect to one another such that an observer cannot, by purely mechanical experiments, distinguish one from the other. Beginning with the behaviour of light (and all other electromagnetic radiation), the theory of special relativity draws conclusions that are contrary to everyday experience but fully confirmed by experiments. Special relativity revealed that the speed of light is a limit that can be approached but not reached by any material object; it is the origin of the most famous equation in science, E = mc²; and it has led to other tantalizing outcomes, such as the “twin paradox.”
“General relativity” is concerned with gravity, one of the fundamental forces in the universe. (The others are electricity and magnetism, which have been unified as electromagnetism, the strong force, and the weak force.) Gravity defines macroscopic behaviour, and so general relativity describes large-scale physical phenomena such as planetary dynamics, the birth and death of stars, black holes, and the evolution of the universe.

Special and general relativity have profoundly affected physical science and human existence, most dramatically in applications of nuclear energy and nuclear weapons. Additionally, relativity and its rethinking of the fundamental categories of space and time have provided a basis for certain philosophical, social, and artistic interpretations that have influenced human culture in different ways.

Cosmology before relativity

The mechanical universe

Relativity changed the scientific conception of the universe, which began in efforts to grasp the dynamic behaviour of matter. In Renaissance times, the great Italian physicist Galileo Galilei moved beyond Aristotle’s philosophy to introduce the modern study of mechanics, which requires quantitative measurements of bodies moving in space and time. His work and that of others led to basic concepts, such as velocity, which is the distance a body covers in a given direction per unit time; acceleration, the rate of change of velocity; mass, the amount of material in a body; and force, a push or pull on a body.
The next major stride occurred in the late 17th century, when the British scientific genius Isaac Newton formulated his three famous laws of motion, the first and second of which are of special concern in relativity. Newton’s first law, known as the law of inertia, states that a body that is not acted upon by external forces undergoes no acceleration—either remaining at rest or continuing to move in a straight line at constant speed. Newton’s second law states that a force applied to a body changes its velocity by producing an acceleration that is proportional to the force and inversely proportional to the mass of the body. In constructing his system, Newton also defined space and time, taking both to be absolutes that are unaffected by anything external. Time, he wrote, “flows equably,” while space “remains always similar and immovable.”
Newton’s laws proved valid in every application, as in calculating the behaviour of falling bodies, but they also provided the framework for his landmark law of gravity (the term, derived from the Latin gravis, or “heavy,” had been in use since at least the 16th century). Beginning with the (perhaps mythical) observation of a falling apple and then considering the Moon as it orbits Earth, Newton concluded that an invisible force acts between the Sun and its planets. He formulated a comparatively simple mathematical expression for the gravitational force; it states that every object in the universe attracts every other object with a force that operates through empty space and that varies with the masses of the objects and the distance between them.

The law of gravity was brilliantly successful in explaining the mechanism behind Kepler’s laws of planetary motion, which the German astronomer Johannes Kepler had formulated at the beginning of the 17th century. Newton’s mechanics and law of gravity, along with his assumptions about the nature of space and time, seemed wholly successful in explaining the dynamics of the universe, from motion on Earth to cosmic events.

Light and the ether

However, this success at explaining natural phenomena came to be tested from an unexpected direction—the behaviour of light, whose intangible nature had puzzled philosophers and scientists for centuries. In 1865 the Scottish physicist James Clerk Maxwell showed that light is an electromagnetic wave with oscillating electrical and magnetic components. Maxwell’s equations predicted that electromagnetic waves would travel through empty space at a speed of almost exactly 3 × 10⁸ metres per second (186,000 miles per second)—i.e., according with the measured speed of light. Experiments soon confirmed the electromagnetic nature of light and established its speed as a fundamental parameter of the universe.
Maxwell’s remarkable result answered long-standing questions about light, but it raised another fundamental issue: if light is a moving wave, what medium supports it? Ocean waves and sound waves consist of the progressive oscillatory motion of molecules of water and of atmospheric gases, respectively. But what is it that vibrates to make a moving light wave? Or to put it another way, how does the energy embodied in light travel from point to point?
For Maxwell and other scientists of the time, the answer was that light traveled in a hypothetical medium called the ether (aether). Supposedly, this medium permeated all space without impeding the motion of planets and stars; yet it had to be more rigid than steel so that light waves could move through it at high speed, in the same way that a taut guitar string supports fast mechanical vibrations. Despite this contradiction, the idea of the ether seemed essential—until a definitive experiment disproved it.
In 1887 the German-born American physicist A.A. Michelson and the American chemist Edward Morley made exquisitely precise measurements to determine how Earth’s motion through the ether affected the measured speed of light. In classical mechanics, Earth’s movement would add to or subtract from the measured speed of light waves, just as the speed of a ship would add to or subtract from the speed of ocean waves as measured from the ship. But the Michelson-Morley experiment had an unexpected outcome, for the measured speed of light remained the same regardless of Earth’s motion. This could only mean that the ether had no meaning and that the behaviour of light could not be explained by classical physics. The explanation emerged, instead, from Einstein’s theory of special relativity.

Special relativity

Einstein’s Gedankenexperiments

Scientists such as Austrian physicist Ernst Mach and French mathematician Henri Poincaré had critiqued classical mechanics or contemplated the behaviour of light and the meaning of the ether before Einstein. Their efforts provided a background for Einstein’s unique approach to understanding the universe, which he called in his native German a Gedankenexperiment, or “thought experiment.”
Einstein described how at age 16 he watched himself in his mind’s eye as he rode on a light wave and gazed at another light wave moving parallel to his. According to classical physics, Einstein should have seen the second light wave moving at a relative speed of zero. However, Einstein knew that Maxwell’s electromagnetic equations absolutely require that light always move at 3 × 10⁸ metres per second in a vacuum. Nothing in the theory allows a light wave to have a speed of zero. Another problem arose as well: if a fixed observer sees light as having a speed of 3 × 10⁸ metres per second, whereas an observer moving at the speed of light sees light as having a speed of zero, it would mean that the laws of electromagnetism depend on the observer. But in classical mechanics the same laws apply for all observers, and Einstein saw no reason why the electromagnetic laws should not be equally universal. The constancy of the speed of light and the universality of the laws of physics for all observers are cornerstones of special relativity.

In developing special relativity, Einstein began by accepting what experiment and his own thinking showed to be the true behaviour of light, even when this contradicted classical physics or the usual perceptions about the world.

The fact that the speed of light is the same for all observers is inexplicable in ordinary terms. If a passenger in a train moving at 100 km per hour shoots an arrow in the train’s direction of motion at 200 km per hour, a trackside observer would measure the speed of the arrow as the sum of the two speeds, or 300 km per hour. In analogy, if the train moves at the speed of light and a passenger shines a laser in the same direction, then common sense indicates that a trackside observer should see the light moving at the sum of the two speeds, or twice the speed of light (6 × 10⁸ metres per second).

While such a law of addition of velocities is valid in classical mechanics, the Michelson-Morley experiment showed that light does not obey this law. This contradicts common sense; it implies, for instance, that both a train moving at the speed of light and a light beam emitted from the train arrive at a point farther along the track at the same instant.
Nevertheless, Einstein made the constancy of the speed of light for all observers a postulate of his new theory. As a second postulate, he required that the laws of physics have the same form for all observers. Then Einstein extended his postulates to their logical conclusions to form special relativity.

Consequences of the postulates

Relativistic space and time

In order to make the speed of light constant, Einstein replaced absolute space and time with new definitions that depend on the state of motion of an observer. Einstein explained his approach by considering two observers and a train. One observer stands alongside a straight track; the other rides a train moving at constant speed along the track. Each views the world relative to his own surroundings. The fixed observer measures distance from a mark inscribed on the track and measures time with his watch; the train passenger measures distance from a mark inscribed on his railroad car and measures time with his own watch.

If time flows the same for both observers, as Newton believed, then the two frames of reference are reconciled by the relation: x′ = x − vt. Here x is the distance to some specific event that happens along the track, as measured by the fixed observer; x′ is the distance to the same event as measured by the moving observer; v is the speed of the train—that is, the speed of one observer relative to the other; and t is the time at which the event happens, the same for both observers. For example, suppose the train moves at 40 km per hour. One hour after it sets out, a tree 60 km from the train’s starting point is struck by lightning. The fixed observer measures x as 60 km and t as one hour. The moving observer also measures t as one hour, and so, according to Newton’s equation, he measures x′ as 20 km.

This analysis seems obvious, but Einstein saw a subtlety hidden in its underlying assumptions—in particular, the issue of simultaneity. The two people do not actually observe the lightning strike at the same time. Even at the speed of light, the image of the strike takes time to reach each observer, and, since each is at a different distance from the event, the travel times differ. Taking this insight further, suppose lightning strikes two trees, one 60 km ahead of the fixed observer and the other 60 km behind, exactly as the moving observer passes the fixed observer. Each image travels the same distance to the fixed observer, and so he certainly sees the events simultaneously. The motion of the moving observer brings him closer to one event than the other, however, and he thus sees the events at different times.

Einstein concluded that simultaneity is relative; events that are simultaneous for one observer may not be for another. This led him to the counterintuitive idea that time flows differently according to the state of motion and to the conclusion that distance is also relative. In the example, the train passenger and the fixed observer can each stretch a tape measure from back to front of a railroad car to find its length. The two ends of the tape must be placed in position at the same instant—that is, simultaneously—to obtain a true value. However, because the meaning of simultaneous is different for the two observers, they measure different lengths.

This reasoning led Einstein to new equations for time and space, called the Lorentz transformations,
after the Dutch physicist Hendrik Lorentz, who first proposed them. They are:

where t′ is time as measured by the moving observer and c is the speed of light.
From these equations, Einstein derived a new relationship that replaces the classical law of addition of velocities,

where u and u′ are the speed of any moving object as seen by each observer and v is again the speed of one observer relative to the other. This relation guarantees Einstein’s first postulate (that the speed of light is constant for all observers). In the case of the flashlight beam projected from a train moving at the speed of light, an observer on the train measures the speed of the beam as c. According to the equation above, so does the trackside observer, instead of the value 2c that classical physics predicts.
To make the speed of light constant, the theory requires that space and time change in a moving body, according to its speed, as seen by an outside observer. The body becomes shorter along its direction of motion; that is, its length contracts. Time intervals become longer, meaning that time runs more slowly in a moving body; that is, time dilates. In the train example, the person next to the track measures a shorter length for the train and a longer time interval for clocks on the train than does the train passenger. The relations describing these changes are

where L₀ and T₀, called proper length and proper time, respectively, are the values measured by an observer on the moving body, and L and T are the corresponding quantities as measured by a fixed observer.

The relativistic effects become large at speeds near that of light, although it is worth noting again that they appear only when an observer looks at a moving body. He never sees changes in space or time within his own reference frame (whether on a train or spacecraft), even at the speed of light. These effects do not appear in ordinary life, because the factor v²/c² is minute at even the highest speeds attained by humans, so that Einstein’s equations become virtually the same as the classical ones.

Relativistic mass

Cosmic speed limit

To derive further results, Einstein combined his redefinitions of time and space with two powerful physical principles: conservation of energy and conservation of mass, which state that the total amount of each remains constant in a closed system. Einstein’s second postulate ensured that these laws remained valid for all observers in the new theory, and he used them to derive the relativistic meanings of mass and energy.
One result is that the mass of a body increases with its speed. An observer on a moving body, such as a spacecraft, measures its so-called rest mass m₀, while a fixed observer measures its mass m as

which is greater than m₀. In fact, as the spacecraft’s speed approaches that of light, the mass m approaches infinity. However, as the object’s mass increases, so does the energy required to keep accelerating it; thus, it would take infinite energy to accelerate a material body to the speed of light. For this reason, no material object can reach the speed of light, which is the speed limit for the universe. (Light itself can attain this speed because the rest mass of a photon, the quantum particle of light, is zero.)

E = mc²

Einstein’s treatment of mass showed that the increased relativistic mass comes from the energy of motion of the body—that is, its kinetic energy E—divided by c². This is the origin of the famous equation E = mc², which expresses the fact that mass and energy are the same physical entity and can be changed into each other.

The twin paradox

The counterintuitive nature of Einstein’s ideas makes them difficult to absorb and gives rise to situations that seem unfathomable. One well-known case is the twin paradox, a seeming anomaly in how special relativity describes time.
Suppose that one of two identical twin sisters flies off into space at nearly the speed of light. According to relativity, time runs more slowly on her spacecraft than on Earth; therefore, when she returns to Earth, she will be younger than her Earth-bound sister. But in relativity, what one observer sees as happening to a second one, the second one sees as happening to the first one. To the space-going sister, time moves more slowly on Earth than in her spacecraft; when she returns, her Earth-bound sister is the one who is younger. How can the space-going twin be both younger and older than her Earth-bound sister?
The answer is that the paradox is only apparent, for the situation is not appropriately treated by special relativity. To return to Earth, the spacecraft must change direction, which violates the condition of steady straight-line motion central to special relativity. A full treatment requires general relativity, which shows that there would be an asymmetrical change in time between the two sisters. Thus, the “paradox” does not cast doubt on how special relativity describes time, which has been confirmed by numerous experiments.

Four-dimensional space-time

Special relativity is less definite than classical physics in that both the distance D and time interval T between two events depend on the observer. Einstein noted, however, that a particular combination of D and T, the quantity D² − c²T², has the same value for all observers.
The term cT in this invariant quantity elevates time to a kind of mathematical parity with space. Noting this, the German mathematical physicist Hermann Minkowski showed that the universe resembles a four-dimensional structure with coordinates x, y, z, and ct representing length, width, height, and time, respectively. Hence, the universe can be described as a four-dimensional space-time continuum, a central concept in general relativity.

Experimental evidence for special relativity

Because relativistic changes are small at typical speeds for macroscopic objects, the confirmation of special relativity has relied on either the examination of subatomic bodies at high speeds or the measurement of small changes by sensitive instrumentation. For example, ultra-accurate clocks were placed on a variety of commercial airliners flying at one-millionth the speed of light. After two days of continuous flight, the time shown by the airborne clocks differed by fractions of a microsecond from that shown by a synchronized clock left on Earth, as predicted.
Larger effects are seen with elementary particles moving at speeds close to that of light. One such experiment involved muons, elementary particles created by cosmic rays in Earth’s atmosphere at an altitude of about 9 km (30,000 feet). At 99.8 percent of the speed of light, the muons should reach sea level in 31 microseconds, but measurements showed that it took only 2 microseconds. The reason is that, relative to the moving muons, the distance of 9 km contracted to 0.58 km (1,900 feet). Similarly, a relativistic mass increase has been confirmed in measurements on fast-moving elementary particles, where the change is large (see below Particle accelerators).
Such results leave no doubt that special relativity correctly describes the universe, although the theory is difficult to accept at a visceral level. Some insight comes from Einstein’s comment that in relativity the limiting speed of light plays the role of an infinite speed. At infinite speed, light would traverse any distance in zero time. Similarly, according to the relativistic equations, an observer riding a light wave would see lengths contract to zero and clocks stop ticking as the universe approached him at the speed of light. Effectively, relativity replaces an infinite speed limit with the finite value of 3 × 10⁸ metres per second.

Compton Effect

Compton Effect 

  

The elastic scattering of electromagnetic radiation by free electrons, accompanied by an increase in wavelength; it is observed during scattering of radiation of short wavelength—X rays and gamma rays. The corpuscular properties of radiation were fully revealed for the first time in the Compton effect. 

The Compton effect was discovered in 1922 by the American physicist A. Compton, who observed that X rays scattered in paraffin have a longer wavelength than the incident rays. Such a shift in wavelength could not be explained by classical theory. In fact, according to classical electrodynamics, under the influence of the periodic electric field of an electromagnetic (light) wave, an electron should oscillate with a frequency equal to that of the wave and consequently should radiate secondary (scattered) waves of the same frequency. Thus, in “classical” scattering (the theory of which was provided by the British physicist J. J. Thomson and is therefore called Thomson scattering) the wavelength of the light does not change. 

An elementary theory of the Compton effect based on quantum concepts was given by Compton and independently by P. Debye. According to quantum theory a light wave is a stream of light quanta, or photons. Each photon has a definite energy ع = hv = hc/λ and a definite momentum pγ = (h/λ)n, where λ is the wavelength of the incident light (v is its frequency), c is the speed of light, h is Planck’s constant, and n is the unit vector in the direction of propagation of the wave (the subscript γ denotes a photon). In quantum theory the Compton effect appears as an elastic collision between two particles, the incident photon and the stationary electron. In every such collision event the laws of conservation of energy and momentum are obeyed. A photon that has collided with an electron transfers part of its energy and momentum to the electron and changes its direction of motion (it is scattered); the decrease in the photon’s energy signifies an increase in the wavelength of the scattered light. The electron, which previously had been stationary, receives energy and momentum from the photon and is set in motion (it experiences recoil). The direction of motion of the particles after the collision, as well as their energy, is determined by the laws of conservation of energy and momentum (Figure 1). 

Figure 1. 

Note to Figure 1. Elastic collision of a photon and an electron in the Compton effect. Before the collision the electron was stationary: pγ and p’γ are the momentum of the incident and scattered photons, pe = mv 

 is the momentum of the recoil electron (v is its velocity), ( is the photon’s scattering angle, and ø is the angle of escape of the recoil electron relative to the direction of the incident photon. 

Simultaneous solution of the equations expressing the equality of the summed energies and momentums of the particles before and after the collision (assuming that the electron is stationary before the collision) gives Compton’s formula for the shift Δλ in the wavelength of the light: 

Δλ = λ’ − λ = λ0(1 ˗ cos θ) 

Here λ’ is the wavelength of the scattered light, θ is the photon’s scattering angle, and λ0 = h/mc = 2.426 × 10˗10 cm = 0.024 angstrom (Å) is the “Compton wavelength” of the electron (m is the mass of the electron). It follows from Compton’s formula that the shift Δλ in the wavelength does not depend on the wavelength λ of the incident light itself. It is solely determined by the scattering angle θ of the photon and is maximal when θ = 180°, that is, when scattering is straight back: Δλmax = 2λo. 

Expressions for the energy عe of the recoil, or “Compton,” electron as a function of the angle ø of its escape may be obtained from the same equations. The dependence of the energy ع’ γ of the scattered photon on the scattering angle θ, as well as the dependence of عe on ø, which is related to it, is shown in Figure 2. From the figure it is apparent that the recoil electrons always have a velocity component in the direction of motion of the incident photon (that is, ø does not exceed 90°). 

Experiment has confirmed all the above theoretical predictions. The correctness of the corpuscular concepts of the mechanism of the Compton effect—and thus the correctness of the basic assumptions of quantum theory—has been experimentally proved. 

In actual experiments on the scattering of photons by matter, the electrons are not free but are bound to atoms. If the energy of the photons is high in comparison with the binding energy of the electrons in the atom (X-ray and gamma-ray photons), then the electrons experience a recoil strong enough to expel them from the atom. In this case the photon scattering proceeds as if with free electrons. However, if the energy of the photon is not sufficient to tear the electron from the atom, then the photon exchanges energy and momentum with the entire atom. Since the mass of the atom is very great compared to the photon’s equivalent mass (which, according to the theory of relativity, equals £y/c2), the recoil is virtually nonexistent; therefore, the photon 

Figure 2. Dependence of the energy ع’λ of the scattered photon on the scattering angle θ (for convenience, only the upper half of the symmetrical curve is depicted) and the dependence of the energy عe of the recoil electron on the angle of escape 0 (lower half of the curve). Quantities related to the same collision event are labeled with identical numbers. The vectors drawn from point 0, at which the collision between the proton with energy عγ and the stationary electron occurred, to corresponding points on the curves depict the state of the particle after scattering: the magnitudes of the vectors give the energy of the particles, and the angles formed by the vectors with the direction of the incident photon define the scattering angle ø and the angle 0 of the recoil electron’s path. (The graph was plotted for the case of scattering of “hard” X rays with wavelength hc/عγ = γo = 0.024 Å.) 

is scattered without a change in its energy (that is, without a change in its wavelength, or “coherently”). In heavy atoms only the peripheral electrons are weakly bound (in contrast to the electrons filling the inner shells of the atom), and therefore the spectrum of the scattered radiation has both a shifted (Compton) line, from scattering by the peripheral electrons, and an un-shifted (coherent) line, from scattering by the entire atom. With increasing atomic number (nuclear charge) the electron binding energy increases, the relative intensity of the Compton line decreases, and that of the coherent line increases. 

The motion of the electrons in atoms leads to a broadening of the Compton lines in the scattered radiation. This occurs because the wavelength of the incident light appears to be slightly changed for moving electrons; in addition, the amount of change depends on the magnitude and direction of the electron’s velocity (the Doppler effect). Careful measurements of the intensity distribution in a Compton line, which reflects the velocity distribution of the electrons in the material, has confirmed the correctness of quantum theory, according to which electrons obey Fermi-Dirac statistics. 

The simplified theory of the Compton effect examined here does not permit the calculation of all characteristics of Compton scattering, particularly the intensity of photon scattering at various angles. A complete theory of the Compton effect is provided by quantum electrodynamics. The intensity of Compton scattering depends on both the scattering angle and the wavelength of the incident radiation. Asymmetry is observed in the angular distribution of the scattered photons: more photons are scattered forward, and the asymmetry increases with increasing energy of the incident photons. The total intensity of Compton scattering decreases with an increase in the energy of the primary photons (Figure 3); this indicates that the probability of the Compton scattering of a photon passing through matter diminishes with decreasing energy. Such a dependence of intensity on £y determines the place of Compton scattering among the other effects of interaction between matter and radiation that are responsible for loss of energy by photons in their passage through matter. For example, in lead the Compton effect makes the main contribution to the energy loss of photons at energies of the order of 1–10 mega electron volts, or MeV (in a lighter element, aluminum, this range is 0.1–30.0 MeV); below this region it is surpassed by the photoelectric effect, and above it by pair production. 

Compton scattering is used extensively in studying the gamma radiation of nuclei; it is also the basis of the principle of operation of some gamma spectrometers. 

The Compton effect is possible not only for electrons but also for other charged particles, such as protons; however, because of the proton’s large mass its recoil is noticeable only during the scattering of photons with very high energy. 

The double Compton effect consists of the formation of two scattered photons in place of a single incident photon during scattering by a free electron. The existence of this process follows from quantum electrodynamics; it was first observed in 1952. Its probability is approximately a hundred times less than that of the ordinary Compton effect. 

Figure 3. Graph showing the dependence of the total Compton scattering intensity <r on the energy of the photon £y (in units of the total intensity of classical scattering); the arrow indicates the energy at which the production of electron-positron pairs begins 

Inverse Compton effect. If the electrons on which electromagnetic radiation is scattered are relativistic (that is, if they are moving with speeds close to the speed of light), then in an elastic collision the wavelength of the radiation will decrease: the energy and momentum of the photons will increase at the expense of the energy and momentum of the electrons. This phenomenon is called the inverse Compton effect and is often used to explain the radiation mechanism of cosmic X-ray sources, the production of the X-ray component of the background galactic radiation, and the transformation of plasma waves into high-frequency electromagnetic waves. 

Compton effect 

The increase in wavelength of electromagnetic radiation, observed mainly in the x-ray and gamma-ray region, on being scattered by material objects. This increase in wavelength is caused by the interaction of the radiation with the weakly bound electrons in the matter in which the scattering takes place. The Compton effect illustrates one of the most fundamental interactions between radiation and matter and displays in a very graphic way the true quantum nature of electromagnetic radiation. Together with the laws of atomic spectra, the photoelectric effect, and pair production, the Compton effect has provided the experimental basis for the quantum theory of electromagnetic radiation. See Angular momentum, Atomic structure and spectra, Electron-positron pair production, Light, Photoemission, Quantum mechanics, Uncertainty principle 

Perhaps the greatest significance of the Compton effect is that it demonstrates directly and clearly that in addition to its wave nature with transverse oscillations, electromagnetic radiation has a particle nature and that these particles, the photons, behave quite like material particles in collisions with electrons. This discovery by A. H. Compton and P. Debye led to the formulation of quantum mechanics by W. Heisenberg and E. Schrödinger and provided the basis for the beginning of the theory of quantum electrodynamics, the theory of the interactions of electrons with the electromagnetic field. 

The Compton effect has played a significant role in several diverse scientific areas. Compton scattering (often referred to as incoherent scattering, in contrast to Thomson scattering or also Rayleigh scattering, which are called coherent scattering) is important in nuclear engineering (radiation shielding), experimental and theoretical nuclear physics, atomic physics, plasma physics, x-ray crystalloghaphy, elementary particle physics, and astrophysics, to mention some of these areas. In addition the Compton effect provides an important research tool in some branches of medicine, in molecular chemistry and solid-state physics, and in the use of high-energy electron accelerators and charged-particle storage rings. 

The development of high-resolution silicon and germanium semiconductor radiation detectors opened new areas for applications of Compton scattering. Semiconductor detectors make it possible to measure the separate probabilities for Rayleigh and Compton scattering. An effective atomic number has been assigned to compounds that appears to successfully correlate theory with Rayleigh-Compton ratios. 

Average density can be measured by moving to higher energies where Compton scattering does not have to compete with Rayleigh scattering. At these energies, Compton scattering intensity has been successfully correlated with mass density. An appropriate application is the measurement of lung density in living organisms. 

The ability to put large detectors in orbit above the Earth' atmosphere has created the field of gamma-ray astronomy. This field is now based largely on the data from the Compton Gamma-Ray Observatory, all of whose detectors made use of the Compton effect (although not exclusively).